The invention relates to a semiconductor device having a photosensitive semiconductor element comprising a semiconductor body having a monocrystalline substrate region of low resistivity, an epitaxial photosensitive layer of compensated semiconductor material of very high resistivity, a first non-rectifying contact on the substrate region, and a second non-rectifying contact on the photosensitive layer.
The above used expression "compensated semiconductor material" means in this context a semiconductor material of very high resistivity which is obtained by compensation as a result of a suitable doping with an element having an active energy level which preferably lies in the vicinity of the centre of the forbidden band of the said semiconductor material. Such a high resistivity compensated material may be of the n type or p type conductivity, according to whether the remaining non-compensated majority charge carriers are electrons or holes.
The invention relates in particular to a photosensitive resistor the photosensitive material of which is gold-compensated silicon.
It is known that the conventional semiconductor photoresistors generally show comparatively large response times (in the order of a few tens of microseconds or even a few milliseconds), in which the response time is defined as the time interval which elapses between the instant at which radiation begins to impinge on the photoresistor and the instant at which said photoresistor reaches the equilibrium value which corresponds to the said radiation.
It has been found, however, that gold-compensated silicon photoresistors may have a response time in the order of one microsecond or less.
A photoresistor of this type is described in the U.S. Pat. specification No. 3,436,613. According to this patent specification, an n type conductive photosensitive layer is formed, by epitaxial growth, on a substrate of a highly doped silicon and is then provided with a likewise highly doped surface layer which is obtained by diffusion. The necessary contacts are provided on the one hand on the substrate and on the other hand on the said surface layer.
The photosensitive layer is compensated with gold by diffusing said metal from the surface of the said surface layer, the electrically active gold concentration being such as to be approximately equal to three times the concentration of the donor atoms in the photosensitive layer.
The response time of a photoresistor obtained according to the above-described structure is approximately 1 microsecond.
However, the great drawback of such a photoresistor is its non-linearity.
It is known that the non-linearity of a photoresistor can be reduced by increasing the thickness of the photosensitive layer of said resistor, but the increase in said thickness has the inevitable drawback that the amplification of the device decreases (it is known that said amplification is inversely proportional to the thickness of the sensitive layer).
On the other hand, a photoresistor in which the gold concentration is significantly larger than the donor concentration of the photosensitive layer and is even equal to three times the said donor concentration, as is the case in the device described here, has a very high and substantially constant value.
On the other hand, Applicants have observed that at lower gold concentrations it is substantially impossible with the structure described to give the dark current resistance the desired value and that the spread of the resulting photoresistors is very large. This is a drawback which hampers the industrial manufacture of said type of photoresistors.
It should also be taken into account that the electric contact on the said surface layer which is considerably disturbed as a result of the gold diffusion does not have the desired quality and that the value of the photoresistor can thus vary with the direction followed by the current flowing through the photoresistor. It has also been found that, due to the poor quality of the said contact, a high offset voltage remains, which means that the value of the voltage extrapolated to a current zero is not equal to zero.
One of the objects of the invention is to prevent or reduce the drawbacks associated with the described known device. The photosensitive semiconductor device according to the invention has a rapid response and a good linearity.
Another object of the invention is to provide a photosensitive semiconductor device having a photosensitive layer of gold-compensated silicon which can be manufactured on an industrial scale and in a reproducible manner.
The invention is based inter alia on the recognition of the fact that the linearity, the possibility of adjusting the desired ohmic base value, and the reproducibility of a photoresistor comprising a compensated photosensitive layer are determined to a considerable extent by the homogeneity of the distribution of the compensating element at the various levels of the structure.
In particular when gold is used as an acceptor for compensating an epitaxial layer of n-conductive silicon and in view of the fact that gold is an element which has a very strong tendency of fixing preferably everywhere where crystal defects occur, for example, at the surface, at interfaces with great disturbances and in diffused zones, the structure and the manufacture of the said device will have to be such that the number of impurity centres is as small as possible.